There is a growing recognition in the scientific community and more broadly
that:

The Earth functions as a system, with properties and behaviour that are
characteristic of the system as a whole. These include critical thresholds,
“switch” or “control” points, strong non-linearities,
teleconnections, chaotic elements, and unresolvable uncertainties. Understanding
the components of the Earth system is critically important, but is insufficient
on its own to understand the functioning of the Earth system as a whole.

Humans are now a significant force in the Earth system, altering key process
rates and absorbing the impacts of global environmental changes. The environmental
significance of human activities is now so profound that the current geological
era can be called the “Anthropocene” (Crutzen and Stoermer, 2000).

A scientific understanding of the Earth system is required to help human societies
develop in ways that sustain the global life support system. The clear challenge
of understanding climate variability and change and the associated consequences
and feedbacks is a specific and important example of the need for a scientific
understanding of the Earth as a system. It is also clear that the scientific
study of the whole Earth system, taking account of its full functional and geographical
complexity over time, requires an unprecedented effort of international collaboration.
It is well beyond the scope of individual countries and regions.

The world’s scientific community, working in part through the three global
environmental change programmes (the International Geosphere-Biosphere Programme
(IGBP), the International Human Dimensions Programme on Global Environmental
Change (IHDP), and the World Climate Research Programme (WCRP)), has built a
solid base for understanding the Earth system. The IGBP, IHDP and WCRP have
also developed effective and efficient strategies for implementing global environmental
change research at the international level. The challenge to IGBP, IHDP and
WCRP is to build an international programme of Earth system science, driven
by a common mission and common questions, employing visionary and creative scientific
approaches, and based on an ever closer collaboration across disciplines, research
themes, programmes, nations and regions.

We need to build on our existing understanding of the Earth system and its
interactive human and non-human processes through time in order to:

improve evaluation and understanding of current and future global change;
and

place on an increasingly firm scientific basis the challenge of sustaining
the global environment for future human societies.

The climate system is particularly challenging since it is known that components
in the system are inherently chaotic, and there are central components which
affect the system in a non-linear manner and potentially could switch the sign
of critical feedbacks. The non-linear processes include the basic dynamical
response of the climate system and the interactions between the different components.
These complex, non-linear dynamics are an inherent aspect of the climate system.
Amongst the important non-linear processes are the role of clouds, the thermohaline
circulation, and sea ice. There are other broad non-linear components, the biogeochemical
system and, in particular, the carbon system, the hydrological cycle, and the
chemistry of the atmosphere.

Given the complexity of the climate system and the inherent multi-decadal time-scale,
there is a central and unavoidable need for long-term consistent data to support
climate and environmental change investigations. Data from the present and recent
past, credible global climate-relevant data for the last few centuries, along
with lower frequency data for the last several millennia, are all needed. Research
observational data sets that span significant temporal and spatial scales are
needed so that models can be refined, validated, or perhaps, most importantly,
rejected. The elimination of models because they are in conflict with climate-relevant
data is particularly important. Running unrealistic models will consume scarce
computing resources, and the results may add unrealistic information to the
needed distribution functions. Such data must be adequate in temporal and spatial
coverage, in parameters measured, and in precision, to permit meaningful validation.
We are still unfortunately short of data for the quantitative assessment of
extremes on the global scale in the observed climate.

In sum, there is a need for:

more comprehensive data, contemporary, historical, and palaeological, relevant
to the climate system;

expanded process studies that more clearly elucidate the structure of fundamental
components of the Earth system and the potential for changes in these central
components;

greater effort in testing and developing increasingly comprehensive and
sophisticated Earth system models;

increased emphasis upon producing ensemble calculations of Earth system
models that yield descriptions of the likelihood of a broad range of different
possibilities, and finally;

new efforts in understanding the fundamental behaviour of large-scale non-linear
systems.

These are significant challenges, but they are not insurmountable. The challenges
to understanding the Earth system including the human component are daunting,
and the pressing needs are significant. However, the opportunity for progress
exists, and, in fact, this opportunity simply must be realised. The issues are
too important, and they will not vanish. The challenges simply must be met.